In today's power networks telecommunication is used to monitor and control the power generation and load flow in the network as well as to provide protection functions which take into account measurements from sensors distributed in the network. For protection purposes, in particular differential protection methods, measurements taken at the two ends of a transmission or a distribution line of a power network are compared with each other. In order to make a measurement taken at the local end truly comparable with a measurement taken at the remote end of the power line, the exact time of actual measuring at the remote end relative to the time of measuring at the local end has to be determined. This is especially important in cases where measurements of an alternating quantity, such as current or voltage of an AC transmission or distribution line, are to be compared with each other and where a fault is detected if the difference between two measurements exceeds a predetermined level. In these cases it is essential to eliminate the part of the difference which is determined only because the measurements are taken at different times, which gives a virtual phase angle difference.
The exact time of measurement at the remote end of the line is achieved by sending out the measurement from the local to the remote end together with a time stamp generated at the time the measurement was taken. The remote end replies with sending out a measurement taken at its own side together with the corresponding time stamp as well as with the timestamp received from the local end. Together with the measurements information is transmitted about the times when the messages were sent and received, respectively. This information is then used in the local end to determine the transmission time, i.e. the time period evolving between the sending and the receiving of a message. The method is known as echo-timing and it is for example described in WO02/061907A1, where it is called “Ping-Pong” technique. Knowing the transmission time, the local end can then recalculate the time when the measurement at the remote side was taken with respect to its own local internal clock. The measuring times of the measurements taken locally and remotely are thereby synchronized with each other so that further processing becomes possible.
The echo-timing method assumes that the transmission time between local and remote end is equal in each direction. This is usually true if the measurements are transmitted via a telecommunication network with fixed transmission routes, i.e. where the messages between two points in the telecommunication network always take the same way. However, modern telecommunication networks are composed of interconnected loops or rings, i.e. they represent a meshed system, and the route a message can take can be freely chosen along the branches of the meshed system. A control unit of such a telecommunication network tries to find the best route across the network based on the current network traffic and transmission quality. This results in unspecified routes and thereby unspecified transmission times between the local and the remote end of the power line. A change from one route to the other is called route switching.
In order to overcome the problem with unspecified transmission times, it is known in the art to add a GPS clock (Global Positioning System) to each end of the power line, as further described in WO02/061907A1. The GPS clocks receive an independent and global time signal from the corresponding satellite system thereby providing a common time frame in the power network. The GPS clocks are used to generate the time stamps for the measurements. The time stamps are thereby automatically synchronized so that further processing of the measurements after transmittal becomes possible without knowledge of the transmission time.
Since the availability of the GPS signal can not be 100%, a back-up solution is required for the periods when GPS is lost. In WO02/061907A1 a method is described, where during GPS reception the transmission times between the local and the remote end are calculated for both directions from the GPS time stamps and stored in a memory. If GPS is lost, the transmission times stored last together with time stamps from the internal clocks are used to align the measurements from local and remote end with each other. Since the method works reliably only as long as the transmission times remain unchanged, i.e. as long as no route switching occurs, it is further suggested to compare the stored transmission times with transmission times determined using the echo-timing method in order to detect a change in the message routing. If a route switching is detected, a fault signal is issued to alert observers that the protection of the power line is no longer reliable.